The present invention relates generally to motorized valve actuators, and in particular, to actuators employing mechanisms which permit the application of a desired close-off force to an associated valve plunger against a valve seat.
In general, motorized valve actuators of the type described herein have been used in applications utilizing fluid conducting pipes, as in heating, ventilating and air conditioning systems and often actuators employed a motorized eccentric cam for providing linear movement of a valve plunger against an associated seat. A problem frequently associated with such mechanisms is the inability to adapt to different applications because of restricted movement of the valve stem. Furthermore, such systems are typically difficult to adjust manually in the event of failure of the motor to which they are attached.
An alternative approach was described in U.S. Pat. No. 4,549,446, issued to the present inventor on Oct. 29, 1985 and assigned to the same assignee as the present invention. The system therein described employed a rack and pinion mechanism for converting rotary motion, supplied by the shaft of the motor, to a linear force for application to the stem of the valve. Additionally, the motorized valve actuator described therein utilized overtravel of an output member relative to the valve seat to create a desired compressive force of the valve plunger against the seat. This resulted, in part, from the use of a coiled spring having a fixed compressive preload disposed between a pair of retaining spring cups. While this design overcame many of the disadvantages of earlier valve actuating systems, it was necessary to provide a different output member, having a different fixed compressive preload, if the particular valve or application desired required a different close-off force against the seat of the valve.
Finally, valve actuating mechanisms of the type described herein have often been difficult to adapt to a wide range of valve designs, sometimes necessitating additional expense as well as time delays in preparing the system for use with a different valve than was previously used. For example, in the '446 patent, described above, the valve was attached to the valve actuating assembly by placing the gland of the valve through an opening in a support bracket associated therewith, and then placing thereon a bushing and a lock nut. Different sized bushings and lock nuts, and possibly different sized brackets are then required depending on the particular valve selected.
These and other shortcomings of past designs are addressed by the valve actuator of the present invention.